Introduction

High production and release of interleukin (IL)-6 from adipose tissue may contribute to dysregulated metabolism in obesity and, thus, contribute to the development of insulin resistance (1–3). Consistently, we recently reported a role of IL-6 signaling in adipocytes in the development of obesity-associated liver insulin resistance and steatosis by making use of adipocyte-specific glycoprotein 130 (gp130) knockout (gp130Δadipo) mice (4). gp130 is a common signal transducer protein of all IL-6–type cytokines that comprises eight different cytokines such as IL-6, IL-11, and leukemia inhibitor factor (LIF) (5). It was previously suggested that these IL-6–type cytokines may affect leptin production in adipose tissue (6–8), thereby potentially mediating a protective effect on glucose metabolism. Interestingly, leptin was found to stimulate the release of glucagon-like peptide 1 (GLP-1) from enteroendocrine cells (9). GLP-1 is an important inducer of glucose-stimulated insulin release in vivo, and it also inhibits gastric emptying and glucagon secretion (10). Hence, identification of factors promoting endogenous GLP-1 release is of interest because enhancing endogenous GLP-1 secretion may be a useful strategy to prevent pancreatic β-cell failure in insulin-resistant obese patients and subsequent development of type 2 diabetes (11). Active GLP-1 is mainly secreted into circulation from intestinal L cells after being cleaved from its precursor proglucagon by the prohormone convertase 1/3 (PC1/3) (10,12,13). The latter is encoded by proprotein convertase subtilisin/kexin type 1 (Pcsk1) that is expressed in intestinal L cells as well as α cells of pacreatic islets (13). Accordingly, GLP-1 can also be produced in pancreatic islets, where it shows paracrine actions (12). In fact, it has recently been proposed that GLP-1 originating from pancreatic islets may be more important for glucose homeostasis than GLP-1 produced in intestinal L cells (14,15).

Results

Impaired Glucose Tolerance in HFD-Fed gp130Δadipo Mice

To investigate a possible role of adipocyte-specific IL-6–type cytokine signaling in glucose metabolism, intraperitoneal glucose tolerance tests were performed in adipocyte-specific gp130 knockout mice (gp130Δadipo) and control littermate mice (gp130F/F) fed a chow or HFD for 12 weeks. As previously shown, gp130 protein levels are reduced in isolated adipocytes but not in skeletal muscle and liver of gp130Δadipo mice compared with control littermates, and body weight (4) as well as food intake was similar in both genotypes (Supplementary Fig. 1). As depicted in Fig. 1A and B, depletion of gp130 in adipocytes led to a stronger deterioration of intraperitoneal glucose tolerance in HFD-fed mice. In contrast, intraperitoneal insulin tolerance was improved in HFD-fed gp130Δadipo mice (Fig. 1C and D), confirming previous findings obtained from hyperinsulinemic-euglycemic clamp studies (4). Such data indicate that impaired glucose tolerance in knockout mice may be the result of blunted GSIS. Indeed, glucose-stimulated circulating insulin levels were significantly lower after intraperitoneal glucose injection in HFD-fed gp130Δadipo compared with gp130F/F mice (Fig. 1E). To investigate whether impaired β-cell function may drive the observed phenotype, GSIS was assessed in islets ex vivo. Of note, GSIS was not impaired in islets isolated from HFD-fed gp130Δadipo mice when compared with control littermates (Fig. 1F). Moreover, insulin content was higher in HFD-fed knockout compared with control mice (Supplementary Fig. 2). Hence, deteriorated glucose-stimulated insulin levels in vivo do not result from defective β-cell function. Rather, impaired incretin secretion may constitute the observed metabolic phenotype.

Recently, different adipose-derived circulating factors were shown to affect GLP-1 release from intestinal cells. In fact, circulating IL-6 was shown to stimulate GLP-1 secretion, thereby enhancing GSIS (26). Moreover, the two adipokines adiponectin and leptin were reported to induce GLP-1 secretion from intestinal L cells (9,27). As depicted in Supplementary Fig. 5, circulating IL-6 and adiponectin levels were similar between HFD-fed gp130F/F and gp130Δadipo mice. In contrast, plasma leptin levels were reduced by ∼40% in gp130Δadipo mice (Fig. 3B) despite similar body weight (4), potentially contributing to their reduced GLP-1 levels. To analyze the impact of IL-6–type cytokine signaling on leptin production in adipocytes, its expression and release were analyzed in isolated adipocytes of chow- and HFD-fed knockout and control mice. As expected, HFD induced leptin mRNA expression in and release from adipocytes of control mice (Fig. 3C and D). Importantly, leptin release from adipocytes was significantly blunted in HFD-fed knockout mice (Fig. 3D), whereas its expression was unchanged (Fig. 3C). In addition, leptin content in isolated adipocytes of HFD-fed mice was similar between the genotypes (Fig. 3E). Hence, IL-6–type cytokine signaling in adipocytes affects leptin release but not its transcription and protein synthesis. We next wanted to investigate which member of the IL-6–type cytokine family mediated the effect on leptin release. To this end, isolated adipocytes of chow-fed gp130F/F and gp130Δadipo mice were incubated with the IL-6–type cytokine family members CNTF, CT-1, IL-6, IL-11, LIF, or OSM, which were previously suggested to affect leptin production or were suggested as potential targets to treat obesity (5–8,28). Although CNTF, CT-1, IL-11, LIF, and OSM had no significant effect on leptin release, IL-6 stimulated leptin secretion in isolated adipocytes gp130 dependently (Fig. 3F). Taken together, IL-6 induced leptin release from adipocytes, thereby contributing to elevated circulating leptin levels in HFD-fed mice.

Reduced GLP-1 release from enteroendocrine cells in adipocyte-specific gp130 knockout mice may result from reduced Pcsk1 expression, the gene encoding PC1/3 controlling GLP-1 production (10,13). Leptin was previously reported to enhance Pcsk1 expression in neuronal cells (30). In HFD-fed gp130Δadipo mice, intestinal Pcsk1 expression was lower compared with control littermates and associated with significantly lower circulating leptin levels. Moreover, supernatant collected from isolated adipocytes regulated Pcsk1 expression in GLUTag cells in a leptin-dependent manner, and recombinant leptin stimulated Pcsk1 expression in GLUTag cells as well as in ileum explants of HFD-fed mice. Of note, the role of intestinal GLP-1 in glucose homeostasis has recently been questioned, suggesting that pancreatic GLP-1 may be more important for glucose homeostasis (14). Indeed, we did not find diminished GLP-1 release from islets isolated from HFD-fed knockout mice. Hence, our data indicate that reduced intestinal rather than pancreatic GLP-1 release affects GSIS and, consequently, glucose homeostasis in gp130Δadipo mice. Although we cannot rule out a role of pancreatic GLP-1, our data clearly support an important role of intestinal GLP-1 production in glucose homeostasis. In support of an involvement of the incretin system in the observed metabolic phenotype in HFD-fed gp130Δadipo mice, insulin release and content were not reduced in islets of HFD-fed knockout mice ex vivo. In fact, insulin content was rather elevated in islets of gp130Δadipo mice, potentially mirroring reduced (GLP-1–stimulated) insulin secretion in vivo. GLP-1 sensitivity of isolated islets was similar between HFD-fed control and knockout mice, suggesting that reduced circulating GLP-1 levels rather than GLP-1 resistance of β-cells cause impaired insulin secretion in knockout mice. In support of such notion, administration of oral glucose, which induces circulating GLP-1 levels in contrast to intraperitoneal glucose injection, exacerbated the difference in glucose tolerance between knockout and control mice.

Lack of IL-6–type cytokine signaling in adipocytes blunts circulating leptin levels independently of fat pad mass (4). Interestingly, reduced circulating leptin levels in knockout mice were paralleled by reduced leptin release from but similar mRNA expression in adipocytes, indicating that gp130 depletion affects leptin secretion but not leptin transcription. Accordingly, it has been suggested that circulating signals regulate leptin on a posttranscriptional level, whereas the long-term nutritional status affects leptin mRNA expression (31). In the past, several IL-6–type cytokines were suggested to affect leptin production or proposed as potential targets to treat obesity (5–8,28). In this study, we suggest that IL-6 contributes to obesity-induced leptin release from adipocytes, adding another metabolic function to the pleiotropic actions of IL-6 (32–34). In fact, among all tested IL-6–type cytokines, only IL-6 significantly increased leptin release from isolated adipocytes. Consistently, treatment of HFD-fed C57BL/6 mice with an anti–IL-6 receptor antibody reduced circulating leptin levels (35). Clearly, we cannot rule out that other IL-6–type family members and/or other molecules binding gp130, such as superantigens (36), contribute to reduced circulating leptin levels in knockout mice. In addition, compensatory changes in the production of other adipose-derived factors in knockout mice may have contributed to reduced leptin levels.

Despite impaired glucose tolerance, insulin sensitivity was improved in HFD-fed gp130Δadipo mice. This observation may be explained by reduced portal free fatty acid (FFA) flux in knockout mice, leading to blunted hepatic insulin resistance and steatosis (4). Hence, IL-6–type cytokine signaling in adipocytes may promote both protective as well as harmful effects on glucose metabolism (Fig. 5). Physiologically, increased IL-6–mediated release of FFA and leptin from adipocytes in obesity may be interpreted as an attempt to reduce lipid stores by increasing lipolysis (FFA release) and by lowering lipid accumulation via leptin-mediated reduction of food intake. Notably, the observed leptin-mediated effect of IL-6 on circulating GLP-1 levels may be additive to the direct effect of IL-6 on GLP-1 release from enteroendocrine cells (26).

Of note, reduced circulating leptin levels in knockout mice did not significantly affect body weight (4), suggesting similar central leptin action between control and knockout mice. In agreement, food intake was similar between HFD-fed gp130F/F and gp130Δadipo littermates. Such a finding may suggest that HFD feeding induces a central leptin resistance, whereas leptin-mediated signaling in the periphery (e.g., in intestinal cells) is less affected (37,38). Accordingly, leptin induced Pcsk1 expression in the ileum of HFD-fed mice. Alternatively, decreased leptin passage across the blood–brain barrier in obese mice and/or local GLP-1 production in the brain (10,37) may lead to unaffected GLP-1 levels in the brain of knockout mice. Clearly, further studies are needed to shed more light on involved mechanisms. In addition, adipocyte-specific depletion of gp130 in female mice may result in a different metabolic phenotype, as leptin secretion and the potency of GLP-1-function are sex dependent (39).

In conclusion, we identify a novel adipo-enteroendocrine axis driven by IL-6 in the regulation of glucose-stimulated insulin release and glucose tolerance in obesity. Such an axis may sensitize pancreatic β-cells to glucose and, thus, counteract adipose tissue-induced insulin resistance (4,40). Moreover, it may offset carbohydrate and/or leptin resistance of enteroendocrine cells, resulting in augmented GLP-1 levels in obesity (9,41–43).

Article Information

Funding. This work was supported by research grants from the Swiss National Science Foundation (310030-160129) and the Gottfried und Julia Bangerter-Rhyner-Stiftung (both to D.K.).

Duality of Interest. No potential conflicts of interest relevant to this article were reported.

Author Contributions. S.W. and D.K. conceived the study and wrote the manuscript. S.W., C.I.L., M.B.-S., F.I., and F.C.L. performed the experimental work. W.M. provided gp130Δadipo mice and gave conceptual advice. M.Y.D. gave conceptual advice. S.W., C.I.L., M.B.-S., F.I., F.C.L., M.B., W.M., M.Y.D., and D.K. contributed to discussion and reviewed and edited the manuscript. D.K. is the guarantor of this work and, as such, had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Prior Presentation. Parts of this study were presented in abstract form at the 77th Scientific Sessions of the American Diabetes Association, San Diego, CA, 9–13 June 2017.